In the field of optical transmissions, it is known to use modulation techniques to send symbols carrying one or more information bits. The modulation techniques make it possible to encode one or more bits over the amplitude and the phase of the field of an optical carrier.
Among modulation techniques, multilevel quadrature amplitude modulation is particularly interesting. Such modulation techniques are called M-QAM for M-quadrature amplitude modulation, where M=2k and k is an integer greater than or equal to 1. The integer k represents the number of bits per symbol.
There are different systems for performing such so-called multilevel amplitude modulations.
It is in particular known to use a modulating system comprising Mach-Zehnder modulators in a so-called I-Q configuration. The modulating system includes an optical power divider with one input to two outputs, each coupled to a Mach-Zehnder modulator, each of the outputs of the Mach-Zehnder modulators being coupled to the two inputs of an optical power combiner (two inputs to one output). The input optical power divider separates an incident optical carrier into two power waves divided by 2, each wave supplying a respective Mach-Zehnder modulator. Each of these two Mach-Zehnder modulators generates a pure amplitude modulation with N levels, N being an integer. The wave propagating in the second Mach-Zehnder modulator is phase shifted relative to the wave propagating in the first Mach-Zehnder modulator using an appropriate phase shifting unit. Thus, two different waves are generated that are phase shifted by π/2 relative to one another, which corresponds to providing two orthogonal components, one being a real component (I) and the other being an imaginary component (Q). These two components are summed at the output coupler. The amplitude of the output signal is modulated with a number of possible states of the signal of 2×N. The obtained optical carrier is modulated by a signal called 2N-QAM. The team of P. DONG et al. demonstrated the feasibility of such a modulation on a silicon Mach-Zehnder modulator to obtain 16 QAM (see in particular the article titled “224-Gb/S PDM-16-QAM Modulator and Receiver based on Silicon Photonic Integrated Circuits”, OFC NOFOEC 2013 paper PDP5C.6).
To generate such 2N-QAM signals, Mach-Zehnder modulators made from lithium niobate are commonly used. Such Mach-Zehnder modulators are commercially available and make it possible to obtain a pure amplitude modulation. With such a device, one skilled in the art knows the relationship between the electrical modulation signal and the amplitude of the optical field, which is a linear relationship once the Mach-Zehnder is powered by an appropriate DC voltage. As a result, the entire modulation space I and Q can be reached.
However, such a modulating system implies, for each Mach-Zehnder modulator, converting a first signal coding a single bit per symbol into a second signal coding several bits per symbol. In order to implement this function, the modulating system for example includes one digital-analog converter per Mach-Zehnder modulator. In practice, each digital-analog converter has imperfections (in particular in terms of resolution) that can cause the deterioration of the overall performance of the modulating system. Furthermore, the Mach-Zehnder modulators have the drawback of being large and consuming large quantities of electricity compared to a resonant ring modulator.
In order to offset these drawbacks, other assemblies have been proposed based on resonant ring modulators, which have the advantage relative to the Mach-Zehnder of being smaller in size and consuming less electricity. Since the ring modulator alone does not allow a pure amplitude modulation, it has been proposed by the same authors as above to use an assembly with two rings instead of Mach-Zehnder modulators forming modulators of the PSK (Phase-Shift Keying) type.
However, such a device is limited to a four-state modulation.
In an article by Y. EHRLICHMAN et al. titled “Generating arbitrary optical signal constellations using microring resonators”, from the journal Optics Express, Volume 21, no. 3, page 3791 to 3799 from February 2013 and in an article by R INTEGLIA et al. titled “Parallel-coupled dual racetrack silicon resonators for quadrature amplitude modulation”, from the review Optics express, volume 19, no. 16 pages 14,892 to 14,902, dated 2011, it is proposed to use to resonant ring modulators in series, the first performing an intensity modulation and the second performing a phase modulation.
However, for the phase modulation, the ring size is several hundreds of microns, which causes a reduction in the modulation speed, which becomes lower than a gigahertz. Furthermore, two digital-analog converters should be used to obtain a 16 QAM modulation.
It therefore appears that all of the systems proposed to date are complex, whether because these systems require a control law that is relatively difficult to generate or because these systems are bulky.